For stretching boat sails, particularly the leech of a mainsail, it is customary to use battens of wooden or plastic slats insertable in pockets in the sail. Both insertion and removal of the battens is time-consuming. Furthermore, it may happen that a hoisted sail must be reefed to provide it with battens which have not been fitted due to carelessness. The battens may also get lost when they are not in use, and sometimes get lost even during use. If the sail provided with the battens is rigged on a boat having a mast and sail implemented such that the sail can be rolled into or round the mast when it is reefed, it is necessary to climb up the mast to remove or fit the battens before rolling in or rolling out the sail.